A water purification device in the related art employs a PP (polypropylene) cotton, an activated carbon or the like as a pre filter cartridge, which is further connected with a RO (reverse osmosis) filter cartridge in series. A water system formed by connecting multi-stage filter cartridges together conducts a water purification treatment. The whole system has a complex pipeline arrangement, and is inconvenient to be mounted and replaced. Moreover, the system has many connectors, which may result in many water leakage risk points. The system needs to be flushed for thirty minutes by a professional installation person when the system is used for the first time or a filter cartridge thereof is replaced. More importantly, the multi-stage filter cartridges have different service lives. A consumer needs to replace 3 to 5 stages of filter cartridges. That is, the consumer needs to purchase the filter cartridges frequently, and also the professional installation person is required to replace the filter cartridge. Thus, the consumer has a poor experience and a comprehensive cost is high.